1. Field of the Invention
The present invention relates to a method for manufacturing a MOS transistor having multi-layered gate insulating film, and more particularly to a method for manufacturing a MOS transistor which can be made smaller with its gate insulating film made thinner and, at the same time, which can keep excellent operating characteristics.
2. Description of the Related Art
Recently, a gate insulating film of a MOS transistor is becoming thinner and thinner as integration density and operating speed become higher. This kind of gate insulating film is usually made up of an oxide film with nitrogen included in it. The reason why nitrogen is included is that the nitrogen atoms prevent impurities in the gate electrodes from diffusing into the oxide film. And, thus, decreasing efficiency of a gate insulating film can be prevented when the film becomes thinner.
Hereafter described are two examples of the conventional manufacturing method for forming a gate insulating film of a MOS transistor of this kind.
As the first example, there is a method of thermal process of heating silicon substrate's regions where MOS transistors are formed in a gas atmosphere including nitrogen atoms, such as nitrogen monoxide (NO) or nitrogen dioxide (NO2) forming a film with SiON. This is a method generally used.
As the second example, conceived is a method of thermal process of heating silicon substrate's regions where MOS transistors are formed, in next processes. That is, these regions are heated, at first, in an oxide gas atmosphere to form oxide film. And, next, these regions are heated in a gas atmosphere including nitrogen atoms to form a film with SiON.
However, the conventional method mentioned above has a problem as follows.
That is, it has been pointed out by the inventor of present invention that nitrogen atoms existing in the oxide film become obstacles for a MOS transistor to operate in good condition, though they prevent impurities in gate electrodes from diffusing. The reason why nitrogen atoms become obstacles for MOS transistor to operate in good condition is conceived that the interface state density of the SiON film is higher than that of the SiO2 film. As a result, passage of charge carriers in the MOS transfer with SiON film is not so smooth as that with SiO2 film. And, this phenomenon becomes more remarkable as a gate insulating film becomes thinner. Therefore, an ideal gate insulating film is a multi-layered insulating film only which upper layer contains nitrogen atoms and which lower layer is a pure oxide film. But, this kind of gate insulating film cannot be obtained by the first conventional manufacturing method mentioned above.
Nor, cannot it be obtained by the second conventional manufacturing method, because nitrogen atoms are segregated into the oxide film in the thermal process. That is, it is vain to try forming upper layer containing nitrogen in an oxide film previously formed in an oxide gas atmosphere by the second method, because nitrogen atoms diffuse into the oxide film according to a phenomenon called segregation. And, nitrogen atoms reach to the bottom of the oxide film when the oxide film is as thin as they can reach there. As a result, what is formed by the second method becomes same as what is formed by the first method. So, this problem becomes more remarkable as the gate insulating film becomes thinner.
Therefore, the present invention is made, concerning especially to a MOS transistor having thin film structure, providing a method for manufacturing a MOS transistor having multi-layered gate insulating film with its upper layer including nitrogen atoms and its lower layer comprising pure oxide.
Meanwhile, the inventor of present invention previously made an invention of method for manufacturing a MOS transistor with thin gate insulating film (U.S. Pat. No. 6,284,580 B1). This method uses segregation process of silicon nitride layer to form a thin gate insulating film. And, lower layer of it is formed of silicon nitride. So, the above problem of not operating in good condition cannot be solved by the invention of this method.